1. Field of the Invention
The present invention relates to a control system and control method for an internal combustion engine equipped with a nitrogen-enriching device in an intake passage for enriching nitrogen in intake air, and an engine control unit.
2. Description of the Related Art
Conventionally, a control system for an internal combustion engine, of the above-mentioned kind, has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H08-254161 (FIG. 1). This control system is applied e.g. to diesel engines, and includes a gas separation device disposed in a branch passage branching from an intake passage. This gas separation device separates incoming intake air (outside air) into nitrogen and oxygen-enriched air by a separation membrane provided in its housing. At an upstream portion of the intake passage, there is disposed a compressor of a turbocharger that supercharges the intake air so as to cause the intake air to pass through the separation membrane. The gas separation device has a nitrogen outlet connected to an intake port via the branch passage, and an oxygen-enriched air outlet connected to cylinders via an oxygen supply passage having an oxygen storage tank interposed therein. A cylinder head of each cylinder has a gas injection valve for injecting oxygen-enriched air into the cylinder. At a location downstream of a branching point where the branch passage branches from the intake passage, there is disposed a butterfly valve. Further, an exhaust gas recirculation (EGR) passage is connected to the branch passage at a location downstream of the gas separation device. In the exhaust gas recirculation passage, there is provided an EGR valve for controlling the amount of EGR gas.
In the control system described above, in a low-load operating region of the engine, for example, the degree of opening of the butterfly valve is controlled such that part of the intake air flows into the branch passage, and the EGR valve is opened, while stopping the operation of the gas injection valves. As a result, nitrogen separated by the gas separation device in the branch passage is supplied to the cylinders, together with outside air through the intake passage, so that the oxygen content of the intake air supplied to the cylinders is reduced as a whole, which provides practical EGR effects. Further, the intake air is depressurized due to pressure loss caused during passing of the intake air through the gas separation device, which promotes recirculation of the EGR gas, so that it is possible to provide proper EGR effects, thereby suppressing generation of NOx.
It is also conventionally known that in a low-load operating condition of a gasoline engine, for example, lean-burn operation is carried out in which the air-fuel ratio of an air-fuel mixture is controlled to be an extremely larger value than the stoichiometric air-fuel ratio in order to improve fuel economy (as disclosed e.g. in the publication of Japanese Patent No. 2817106). The effect of improving fuel economy by this lean-burn operation is achieved through reduction of heat loss by an increase of the working fluid and the improvement of combustion efficiency by reduction of pumping loss, and as shown in FIG. 9, when the air-fuel ratio is approximately 21.0, the brake specific fuel consumption BSFC is minimized, whereby the best fuel economy can be achieved.
However, in the lean-burn operation described above, although fuel economy is improved, NOx in exhaust gases cannot be reduced sufficiently by a three-way catalyst generally used as an emission reduction device. This is because in lean-burn operation, the air-fuel ratio of an air-fuel mixture is higher than the stoichiometric air-fuel ratio, so that a large amount of superfluous oxygen remains unburnt in the exhaust gases, whereas the three-way catalyst exhibits its highest performance for reducing CO, HC and NOx, under a condition where oxygen density is substantially zero.
To solve this problem, conventionally, as a substitute for the three-way catalyst, there have been proposed the following emission reduction devices: (1) a catalyst of a type that stores NOx during lean-burn operation and reduces NOx during stoichiometric combustion executed at the stoichiometric air-fuel ratio; (2) a catalyst of a type capable of reducing NOx even when superfluous oxygen exists. However, these catalysts are both more expensive than the three-way catalyst. Further, in the case of the first type, when storage of NOx reaches saturation, lean-burn operation has to be interrupted so as to prevent NOx from being emitted, and hence the effect of improving fuel economy by lean-burn operation cannot be fully obtained. On the other hand, the second type is not capable of achieving excellent emission control since its NOx reduction rate is low when superfluous oxygen exists, which prevents this type from becoming commercially practical.
Since the problem of NOx reduction in lean-burn operation results from the existence of superfluous oxygen as described above, it is effective to make use of the gas separation device disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 8-254161 (FIG. 1 of the publication), to supply nitrogen from the gas separation device to the engine during lean-burn operation. In this control system, however, since the butterfly valve is opened only to allow part of intake air to flow into the gas separation device, for supply of nitrogen, the amount of intake air that actually flows into the gas separation device cannot be gasped accurately. Further, a gas separation device of this kind has the characteristic that its separation performance varies with the boost pressure and the temperature of intake air, which makes the degree of separation between nitrogen and oxygen-enriched air unknown, and the separation performance is degraded due to insufficiency of boost pressure e.g. at the start of the turbocharger. Consequently, the above control system cannot accurately control the actual amount of nitrogen in intake air supplied to the cylinders of the engine, i.e. the actual amount or concentration of oxygen, and therefor superfluous oxygen inevitably remains in exhaust gases. As a result, the emission-reducing potential of the three-way catalyst cannot be fully exploited, which also makes it impossible to achieve excellent exhaust emission control.
Further, in this control system, in low-load operation, EGR is carried out simultaneously with supply of nitrogen. For this reason, CO2 with a high specific heat is increased by combustion, causing increased heat loss. As a result, combustion efficiency is reduced, which makes it impossible to sufficiently obtain the effect of improving fuel economy by lean-burn operation. Moreover, CO in the EGR gas can cause carbon clogging and generation of smoke.
It is an object of the present invention to provide a control system and control method for an internal combustion engine and an engine control unit which are capable of meeting a driver""s demand of torque, and achieving high combustion efficiency and high emission reducing performance by a three-way catalyst in lean-burn operation in a compatible manner.
To attain the above object, in a first aspect of the present invention, there is provided a control system for an internal combustion engine including a throttle valve and a nitrogen-enriching device that enriches nitrogen in intake air, both disposed in an intake passage, and an emission reduction device disposed in an exhaust passage, for reducing exhaust emissions,
the control system comprising:
an oxygen concentration sensor provided in the exhaust passage, for detecting oxygen concentration in exhaust gases;
control amount-setting means for setting a control amount indicative of one of a mass of oxygen supplied to a combustion chamber of the engine and a fuel injection amount, such that the detected oxygen concentration becomes equal to an oxygen concentration corresponding to stoichiometric combustion;
operating condition-detecting means for detecting operating conditions of the engine;
target value-setting means for setting a target value of the control amount, which corresponds to a torque demanded of the engine, based on the detected operating conditions of the engine; and
throttle valve opening control means for controlling a degree of opening of the throttle valve such that the control amount becomes equal to the set target value.
In the internal combustion engine, intake air (outside air) having flowed into the intake passage has nitrogen therein enriched (or oxygen therein reduced) by the nitrogen-enriching device, and then the intake air is supplied to the combustion chamber. The amount of intake air is controlled by the throttle valve. Further, with the arrangement of this control system, the control amount indicative of one of the mass of oxygen supplied to a combustion chamber of the engine and the fuel injection amount is set such that the oxygen concentration in the intake air detected by the oxygen concentration sensor becomes equal to an oxygen concentration corresponding to stoichiometric combustion (i.e. combustion in which fuel is burned in a state containing a just enough amount of oxygen, without producing substantially any superfluous oxygen). This configuration makes it possible to control the ratio between the mass of oxygen actually supplied to the combustion chamber and the fuel injection amount, such that stoichiometric combustion without substantially any superfluous oxygen takes place. Further, since the degree of opening of the throttle valve is controlled such that the control amount becomes equal to a target value corresponding to a demanded torque, it is possible to meet a driver""s demand of torque.
As described above, intake air containing enriched nitrogen is supplied to the combustion chamber, and at the same time the amount of intake air and the fuel injection amount are controlled as above, whereby in an operating condition where torque demand is low, it is possible to perform lean-burn operation in which the air-fuel ratio is controlled to a far larger value than the stoichiometric air-fuel ratio, in the state of stoichiometric combustion. As a result, even in lean-burn operation, for example, exhaust gases can be held in the stoichiometric state containing hardly any superfluous oxygen, and therefore, even when a three-way catalyst is used as an emission reduction device, it is possible to fully exploit the emission-reducing potential thereof, thereby achieving excellent emission control.
Nitrogen-enriched air is generated using the nitrogen-enriching device by replacing a portion of an oxygen component in outside air (hereinafter referred to as xe2x80x9cnatural airxe2x80x9d for distinction from nitrogen-enriched air) with a nitrogen component. Further, since oxygen and nitrogen are 2-atom molecules, they are equal in specific heat, and hence they are also equal in heat loss in combustion. Therefore, even when nitrogen-enriched air is used in lean-burn operation, it is possible to maintain as high combustion efficiency and excellent fuel economy as in lean-burn operation using natural air. Thus, the present invention makes it possible to achieve high combustion efficiency and high emission reducing performance by a three-way catalyst in lean-burn operation in a compatible manner while meeting a driver""s demand of torque. Further, differently from the case of using EGR, there is no possibility of causing carbon clogging or smoke since nitrogen-enriched air contains no impurity of CO.
Preferably, a rate of nitrogen enrichment by the nitrogen-enriching device is set such that the oxygen concentration in the intake air to be supplied to the combustion chamber becomes equal to a predetermined oxygen concentration.
The composition of natural air is: N:CO=79% : 21% (see FIG. 10(a)), and in a case where natural air is used, stoichiometric combustion takes place when the air-fuel ratio is a stoichiometric air-fuel ratio (air:fuel=14.7:1.0). On the other hand, when lean-burn operation is performed using natural air as described hereinbefore, due to improved combustion efficiency, best fuel economy is achieved at an air-fuel ratio of around 21.0 (hereinafter referred to as xe2x80x9cthe best-fuel economy air-fuel ratioxe2x80x9d)(see FIG. 9), but since oxygen is excessive with respect to fuel, superfluous oxygen occurs (see FIG. 10(c)). Further, with the arrangement of the present invention, through the control of the mass of oxygen and the fuel injection amount, the state of stoichiometric combustion is maintained even in lean-burn operation, as described hereinbefore. From the above relationship, it is considered that an oxygen concentration X in intake air for achieving best fuel economy while maintaining the state of stoichiometric combustion in lean-burn operation is: X=21%xc3x9714.7/21.0=14.7% (see FIG. 10(b)). Therefore, according to this preferred embodiment, by setting the rate of nitrogen enrichment of the nitrogen-enriching device such that the oxygen concentration in intake air becomes equal to the predetermined concentration (=14.7%, hereinafter referred to as xe2x80x9cthe optimal concentrationxe2x80x9d), it is possible to maintain high emission-reducing performance by the three-way catalyst in lean-burn operation, and at the same time by controlling the air-fuel ratio to the best-fuel economy air-fuel ratio, it is possible to achieve the most excellent combustion efficiency and the best fuel economy.
Preferably, the throttle valve comprises a main throttle valve provided in the intake passage at a location downstream of the nitrogen-enriching device and a sub-throttle valve provided in a bypass passage bypassing the nitrogen-enriching device and joining the intake passage at a location upstream of the main throttle valve, and the throttle valve opening control means controls a degree of opening of the main throttle valve and a degree of opening of the sub-throttle valve.
With the arrangement of this preferred embodiment, by controlling the degree of opening of the main throttle valve, it is possible to control the amount of intake air to be supplied to the combustion chamber of the engine. Further, by controlling the degree of opening of the sub-throttle valve, it is possible to change the ratio between the amount of nitrogen-enriched air from the nitrogen-enriching device and the amount of natural air from the bypass passage. This makes it possible to control the oxygen concentration in intake air to be supplied to the combustion chamber, as desired. Therefore, by controlling the degree of opening of the main throttle valve and the degree of opening of the sub-throttle valve such that the control amount becomes equal to a target value corresponding to a demanded torque, it is possible to obtain the optimal amount of intake air and optimal oxygen concentration corresponding to the demanded torque of the engine at the time. For example, in lean-burn operation with a small demanded torque, by controlling the degree of opening of each of the two throttle valves such that the amount of intake air increases and at the same time the oxygen concentration decreases, it is possible to achieve stoichiometric combustion, whereas in a high output operating condition, by controlling the degree of opening of each of the two throttle valves such that both the amount of intake air and the oxygen concentration increase, it is possible to meet a high torque demand.
More preferably, the control system further comprises intake air amount-detecting means for detecting an amount of intake air supplied to the combustion chamber, and intake air amount-determining means for determining whether or not the detected amount of intake air has reached a predetermined upper limit intake air amount, and when the detected amount of the intake air has not reached the predetermined upper limit intake air amount, the throttle valve opening control means holds the degree of opening of the sub-throttle valve at a predetermined appropriate degree of opening and controls the degree of opening of the main throttle valve.
With the arrangement of this preferred embodiment, the degree of opening of the sub-throttle valve is held at the predetermined appropriate degree of opening until the amount of intake air reaches the predetermined upper limit intake air amount, and at the same time the degree of opening of the main throttle valve is controlled such that the control amount becomes equal to the target value. As a result, the amount of intake air is increased or decreased according to a demanded torque, with the oxygen concentration in the intake air being held substantially constant, whereby a mass of oxygen commensurate with the demanded torque is supplied to the combustion chamber. Further, it is possible to carry out lean-burn operation until the amount of intake air reaches the predetermined upper limit intake air amount, i.e. to the limit within which the demanded torque can be met. This makes it possible to further improve fuel economy.
Further preferably, the predetermined appropriate degree of opening of the sub-throttle valve is set such that the concentration of oxygen in the intake air to be supplied to the combustion chamber becomes equal to a predetermined concentration.
With the arrangement of this preferred embodiment, since the predetermined appropriate degree of opening of the sub-throttle valve is set as above, before the amount of intake air reaches the predetermined upper limit intake air amount, the degree of opening of the sub-throttle valve is held at the predetermined appropriate degree of opening, whereby the concentration of oxygen in the intake air to be supplied to the combustion chamber is controlled such that it is substantially equal to the predetermined concentration. Therefore, by setting the aforementioned optimal concentration to this predetermined concentration, it is possible to achieve the most excellent combustion efficiency and the best fuel economy in lean-burn operation.
Further preferably, after the detected amount of intake air has reached the predetermined upper limit intake air amount, the throttle valve opening control means holds the degree of opening of the main throttle valve at a predetermined effective degree of opening and controls the degree of opening of the sub-throttle valve.
With the arrangement of this preferred embodiment, after the amount of intake air has reached the predetermined upper limit intake air amount, the degree of opening of the main throttle valve is held at the predetermined effective degree of opening, and the degree of opening of the sub-throttle valve is controlled such that the control amount becomes equal to the target value. As a result, the oxygen concentration in intake air is increased and decreased while holding the amount of intake air substantially constant, whereby a mass of oxygen commensurate with a demanded torque is supplied to the combustion chamber. Therefore, even in high output operation, it is possible to meet a high torque demand, and maintain high emission-reducing performance by the three-way catalyst since the state of stoichiometric combustion is maintained. Further, in this case, by setting the flow rate of intake air such that when the sub-throttle valve is fully open, the major portion of the intake air flows into the bypass passage, and the effect of nitrogen enrichment by the nitrogen-enriching device can hardly be obtained, it is possible to achieve substantially as high a torque as is obtained when natural air is used.
Even more preferably, the predetermined effective degree of opening of the main throttle valve is set to such a degree of opening that even if the degree of opening of the main throttle valve is further increased from the predetermined effective degree of opening, the amount of intake air does not increase any further.
With the arrangement of this preferred embodiment, since the predetermined effective degree of opening of the main throttle valve is set as above, after the amount of intake air has reached the predetermined upper limit intake air amount, by holding the degree of opening of the main throttle valve at the predetermined effective degree of opening, it is possible to secure the maximum intake air amount, and hence it is also possible to sufficiently meet a high torque demand.
Preferably, the control system further comprises a supercharger provided in the intake passage at a location upstream of the nitrogen-enriching device, for supercharging the intake air.
In general, the nitrogen-enriching device has a membrane provided therein, and performs enrichment of nitrogen by passing air through the membrane. With the arrangement of this preferred embodiment, differential pressure for causing air to pass through the membrane can be secured by supercharging of intake air by the supercharger, whereby it is possible to obtain a sufficient nitrogen enrichment rate. When the supercharger is implemented by a compressor of a turbocharger, boost pressure is insufficient e.g. at the start-up of the engine, so that the nitrogen enrichment rate tends to become insufficient. However, according to this preferred embodiment, exhaust gases can be held in the state of stoichiometric combustion irrespective of the nitrogen enrichment rate, as described hereinbefore, so that it is possible to obtain the above described advantageous effects without difficulty.
To attain the above object, in a second aspect of the present invention, there is provided a method of controlling an internal combustion engine including a throttle valve and a nitrogen-enriching device that enriches nitrogen in intake air, both disposed in an intake passage, and an emission reduction device disposed in an exhaust passage, for reducing exhaust emissions,
the method comprising the steps of:
detecting oxygen concentration in exhaust gases;
setting a control amount indicative of one of a mass of oxygen supplied to a combustion chamber of the engine and a fuel injection amount, such that the detected oxygen concentration becomes equal to an oxygen concentration corresponding to stoichiometric combustion;
detecting operating conditions of the engine;
setting a target value of the control amount, which corresponds to a torque demanded of the engine, based on the detected operating conditions of the engine; and
controlling a degree of opening of the throttle valve such that the control amount becomes equal to the set target value.
With the arrangement of the second aspect of the present invention, it is possible to obtain the same advantageous effects as provided by the first aspect of the present invention.
Preferably, the throttle valve comprises a main throttle valve provided in the intake passage at a location downstream of the nitrogen-enriching device and a sub-throttle valve provided in a bypass passage bypassing the nitrogen-enriching device and joining the intake passage at a location upstream of the main throttle valve, and the step of controlling a degree of opening of the throttle valve includes controlling a degree of opening of the main throttle valve and a degree of opening of the sub-throttle valve.
More preferably, the method further comprises a step of detecting an amount of intake air supplied to the combustion chamber, and a step of determining whether or not the detected amount of intake air has reached a predetermined upper limit intake air amount, and the step of controlling a degree of opening of the throttle valve includes holding the degree of opening of the sub-throttle valve at a predetermined appropriate degree of opening and controlling the degree of opening of the main throttle valve, when the detected amount of the intake air has not reached the predetermined upper limit intake air amount.
Further preferably, the predetermined appropriate degree of opening of the sub-throttle valve is set such that the concentration of oxygen in the intake air to be supplied to the combustion chamber becomes equal to a predetermined concentration.
Further preferably, the step of controlling a degree of opening of the throttle valve includes holding the degree of opening of the main throttle valve at a predetermined effective degree of opening and controlling the degree of opening of the sub-throttle valve, after the detected amount of intake air has reached the predetermined upper limit intake air amount.
Even more preferably, the predetermined effective degree of opening of the main throttle valve is set to such a degree of opening that even if the degree of opening of the main throttle valve is further increased from the predetermined effective degree of opening, the amount of intake air does not increase any further.
Preferably, the engine includes a supercharger provided in the intake passage at a location upstream of the nitrogen-enriching device, for supercharging the intake air.
With the arrangements of these preferred embodiments, it is possible to obtain the same advantageous effects as provided by the corresponding preferred embodiments of the first aspect of the present invention.
To attain the above object, in a third aspect of the present invention, there is provided an engine control unit including a control program for causing a computer to control an internal combustion engine including a throttle valve and a nitrogen-enriching device that enriches nitrogen in intake air, both disposed in an intake passage, and an emission reduction device disposed in an exhaust passage, for reducing exhaust emissions,
wherein the control program causes the computer to detect oxygen concentration in exhaust gases, set a control amount indicative of one of a mass of oxygen supplied to a combustion chamber of the engine and a fuel injection amount, such that the detected oxygen concentration becomes equal to an oxygen concentration corresponding to stoichiometric combustion, detect operating conditions of the engine, set a target value of the control amount, which corresponds to a torque demanded of the engine, based on the detected operating conditions of the engine, and control a degree of opening of the throttle valve such that the control amount becomes equal to the set target value.
With the arrangement of the third aspect of the present invention, it is possible to obtain the same advantageous effects as provided by the first aspect of the present invention.
Preferably, the throttle valve comprises a main throttle valve provided in the intake passage at a location downstream of the nitrogen-enriching device and a sub-throttle valve provided in a bypass passage bypassing the nitrogen-enriching device and joining the intake passage at a location upstream of the main throttle valve, and when the control program causes the computer to control the degree of opening of the throttle valve, the control program causes the computer to control the degree of opening of the main throttle valve and the degree of opening of the sub-throttle valve.
More preferably, the control program causes the computer to detect an amount of intake air supplied to the combustion chamber, and determine whether or not the detected amount of intake air has reached a predetermined upper limit intake air amount, and when the control program causes the computer to control the degree of opening of the throttle valve, the control program causes the computer to hold the degree of opening of the sub-throttle valve at a predetermined appropriate degree of opening and control the degree of opening of the main throttle valve, if the detected amount of the intake air has not reached the predetermined upper limit intake air amount.
Further preferably, the predetermined appropriate degree of opening of the sub-throttle valve is set such that the concentration of oxygen in the intake air to be supplied to the combustion chamber becomes equal to a predetermined concentration.
Further preferably, when the control program causes the computer to control the degree of opening of the throttle valve, the control program causes the computer to hold the degree of opening of the main throttle valve at a predetermined effective degree of opening and control the degree of opening of the sub-throttle valve, after the detected amount of intake air has reached the predetermined upper limit intake air amount.
Even more preferably, the predetermined effective degree of opening of the main throttle valve is set to such a degree of opening that even if the degree of opening of the main throttle valve is further increased from the predetermined effective degree of opening, the amount of intake air does not increase any further.
Preferably, the engine includes a supercharger provided in the intake passage at a location upstream of the nitrogen-enriching device, for supercharging the intake air.
With the arrangements of these preferred embodiments, it is possible to obtain the same advantageous effects as provided by the corresponding preferred embodiments of the first aspect of the present invention.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.